There are two reports you should run to verify your data is correct before sending reviewers an email to sign in and start reviewing submissions:
ROLE COUNT REPORT
Go to Reports People
Go to the 'Speaker Reports' header and click on the 'Role Count Report'
Under the 'Assigned People' header, filter by Assigned Role = Reviewer and click 'Run Report'
Confirm your reviewers are assigned equally to submissions (or according to your business rules)
SUBMISSION REVIEWER STATUS
Go to Reports Submissions
Under the 'Forms Report' header, click on the 'Submission Reviewer Status (By Submission)' report
Under the 'Speaker' header, filter by Person Role = Reviewer and make sure the correct review form is selected under 'Step 2 - Title'
Confirm your submissions have an equal amount of reviewers assigned (or according to your business rules)